Tank with integral remotely controlled power actuated bottom valve

ABSTRACT

A tank and integral valve assembly suitable for storing and controlling the transfer of viscous fluids. The assembly utilizes a vessel in which is constructed a butterfly-type valve assembly located so as to regulate rate of egress of viscous fluid from the bottom of the vessel. An actuator assembly associated with a lateral side portion of the vessel controls rotary movements of a valve disc. The actuator is responsive to a level sensor coupled thereto through a controller assembly.

United States/Patent [191 Gordon a July 16, 1974 TANK WITH INTEGRAL REMOTELY CONTROLLED POWER ACTUATED BOTTOM VALVE [52] US. Cl 137/395, 137/571, 251/144, 251/298, 251/305, 222/556 [51] Int. Cl. F16k 31/02 [58] Field of Search 137/173, 174, 255, 256, 137/259, 263, 264, 386, 391, 393, 395,

2,694,515 11/1954 Green 222/556 X 3,362,337 l/l968 3,446,222 5/1969 3,653,543 4/1972 Preikschat 222/64 X Primary Examiner--William R. Cline Assistant Examiner-David R. Matthews Attorney, Agent, or Firm-Joseph S. Nelson; Edwa P. Gratton; James C. Logomasini [5 7] ABSTRACT A tank and integral valve assembly suitable for storing and controlling the transfer of viscous fluids. The assembly utilizes a vessel in which is constructed a butterfly-type valve assembly located so as to regulate rate of egress of viscous fluid from the bottom of the vessel. An actuator assembly associated with a lateral side portion of the vessel controls rotary movements of a valve disc. The actuator is responsive to a level sensor coupled thereto through a controller assembly.

3 Claims, 4 Drawing Figures [56] References Cited NITEQQTAIELPAIENTS 2,729,238 l/1956 Hite 251/305 X TANK WITH INTEGRAL REMOTELY CONTROLLED POWER ACTUATED BOTTOM VALVE BACKGROUND In the chemical industry, it is sometimes necessary to discharge fluid material from one tank into a second tank located below the first tank for temporary storage or holding on a batch or continuous basis in a controlled manner and at a controlled rate. When the fluid material is heated and viscous, the problem of controlled material transfer from one tank to the second can become very severe, particularly when gravitational forces are to be used at least in part to provide the power causng such material to flow from the one to the second tank.

For example, in the falling strand devolatilization of a polymer syrup such as a polystyrene/styrene monomer mixture, it has heretofore been relatively common practice to preheat such syrup, as in a shell and tube preheater, from an already melted condition to a higher temperature and then to discharge the so-heated syrup into a flash chamber (or flash tank) wherein the temperature/pressure relationship is above the boiling point of the styrene monomer but below that of the homopolystyrene polymer so that the monomer is separated from the polymer and removed from the flash chamber as a vapor. Separated monomer vapor is commonly removed from a location near the top of the flash chamber while the polymer, which is in the form of a hot, viscous melt, is pumped or drained from a bottom region of the flash chamber for further processing.

In order to remove as much of the monomer as possible from the polymer, it was and is commonly to subject the polymer hot melt recovered from a first flash chamber to another sequence of (second) preheater heating followed by passage through a (second) flash chamber, the second preheater generally being at a higher temperature, and the second flash chamber being at a higher temperature and lower pressure than the preceding first-stage of heating/flashing. Recently,

an improvement in this falling strand devolatilization procedure has been discovered by which the need for the second preheater has been eliminated and hot melt from a first flash chamber is passed directly into a second flash chamber. Such improved procedure requires control of hot melt transfer from first into second flash chamber not only to regulate flow rate, but also to permit maintenance of a pressure differential between the two chambers. Specifically it is necessary to maintain the hot melt level in the bottom of a first flash tank at some predetermined level and to meter such hot melt from this flash tank into a second flash tank continuously and at a controlled rate.

Such a level maintenance and metering can be accomplished by a pump or valve means. However, owing to the combination of elevated temperatures, pressure differential between the first flash tank and the second flash tank, viscosity of the hot melt fluids involved, rate of hot melt flow from first flash tank into second flash tank, and the like, I have found that a major problem exists in accomplishing hot melt transfer directly from such a first into such a second flash tank. This problem' is particularly important in the type of continuous process operation involved in the continuous falling strand devolatilization of a polymer syrup such as a polystyrene/styrene monomer mixture where long periods of continuous, trouble-free operation with a high degree of reliability are contemplated. Furthermore, such an operation should be conducted with relatively simple, relatively low cost, controlled, hot melt transfer means.

SUMMARY I have now discovered a new and very useful apparatus to store and control the transfer of viscous fluid from a process tank to a subsequent processing operation. This invention is further adapted for use in transferring a hot, viscous fluid from the bottom region of a process tank to a spatially lower process station under conditions partly dependent upon gravitational forces, and this invention is particularly well adapted for transferring such a fluid fromsuch a tank to a second tank which is normally maintained during equipment operation at a pressure lower than that of the (first) process tank.

The invention utilizes relatively simple, easy to maintain components, and provides the capability of operation continuously for extended periods of time with a high degree of reliability.

The invention may be used more or less as a separate system in practicing some chemical or physical operation upon fluid materials, or it may be used in combination with other elements as a sub-combination of a greater system. For example, in this last connection, the invention may be used as part of a falling-strand devolatilizer which employs a preheater preceding two subsequent flash chambers wherein the present invention is utilized as the fluid transfer arrangement. Those skilled in the art of chemical process industry techniques, however, will readily appreciate that the present invention is useful in many applications.

Briefly, the present invention employs as one element a generally enclosed vessel having adjacent one end region thereof a generally funnel shaped or tapered configuration, the vessel being adapted to be oriented spatially with such tapered end region downwardly extending. The vessel has a port defined in the apex portion of this tapered end section. The vessel may have additional ports depending on the end use application contemplated, as desired by a particular end user.

A valve disc means is positioned generally across said valve plate means. This valve disc means is adapted for rotational movements about a generally horizontally extending axis in a generally cylindrical opening. This opening may be part of the tank assembly or part of a separate attached valve body or valve plate means. This valve disc means is further adapted to substantially shut the opening within which such disc rotates when such disc is generally in a horizontal position in an operating embodiment.

A valve plate means if used is mounted in and about the opening within which the valve disc means rotates. The valve plate may be integral with and form a part of the construction material used to make the vessel, or otherwise, as those skilled in the art will appreciate.

A valve stem means extends generally coaxially with the aforeindicated horizontal axis, and the valve stem means projects beyond opposed side edges of the valve disc means through adjacent openings defined in the aforesaid valve plate means. Such valve stem means can be considered to include bearing means and this valve stem means is adapted to rotatably move the valve disc means about the indicated horizontal axis between shut and open positions relative to the apparatus in the valve plate means. One end portion of the valve stem means project laterally outwardly to a terminal location exteriorly located with respect to the vessel.

A variable actuator means is exteriorly located with respect to the vessel and is functionally engaged with the aforeindicated exposed terminal end region of the valve stem means. This actuator means is adapted in response to signal means applied thereto to move revolvably the valve stem means relative to the vessel.

Level sensing means is associated with the vessel. Such level sensing means is adapted both to measure a fluid level in the vessel and to generate a signal means in response thereto when the vessel is functioning with the valve equipped port downwardly oriented spatially.

A controller means functionally interconnects the level sensing means and the actuator means. The controller means is adapted in response to signals from the level sensing means to generate control signals for the actuator means to control rotational movements of the valve stem means, and the valve disc means associated therewith, in a predetermined manner so that a predetermined variable aperture is maintainable between the valve disc means and the valve plate means, thereby to regulate the flow of fluid material moving therebetween when the vessel is functioning with a port downwardly positioned.

FIGURE DESCRIPTION The invention is better understood by reference to the attached drawing wherein:

FIG. 1 is a side elevational view of one falling strand devolatilization apparatus incorporating an embodiment of the present invention, some parts thereof broken away and some parts thereof shown in section;

FIG. 2 is a block diagrammatic view of a level control system suitable for use with the apparatus of FIG. 1;

FIG. 3 is a side elevational view of another falling strand devolatilizer apparatus incorporating another embodiment of the present invention, some parts thereof broken away and some parts thereof shown in section; and

FIG. 4 is a vertical sectional view illustrating details of the valve assembly used in the apparatus of FIG. 3, some parts thereof broken away and some parts thereof shown in section.

DETAILED DESCRIPTION Turning to FIG. 1 there is seen illustrated a first embodiment of this invention herein designated in its entirely by the numeral 10. As shown in FIG. 1, embodiment is incorporated into a falling strand devolatilizer apparatus herein designated in its entirety by the prise a shell and tube heat exchanger assembly 12, a

vessel 14 or first flash tank which comprises part of the Vessel 14 has a tapered lower region 16 which terminates in a discharge port 17. Vessel 14 also has a vapor take-off port formed by pipe and flange assembly 18.

' The assembly 18 communicates with the interior space numeral 11. Devolatilizer 11 can be considered to com- 19 of vessel 14.

Vessel 14 has a double walled, jacketed construction in its upper portion 15 for purposes of controlling the interior temperatures of vessel 14 during operation of the devolatilizer 11.

Vessel 14 is integral with tank 13; thus, the wall of tapered lower portion 16 of vessel 14 forms the top wall of tank 13. The side walls 21 and tapered lower region 22 of tank 13 are of double walled jacketed construction for purposes of controlling the interior temperatures of the vessel 13 during operation of the devolatilizer 11. A vapor take-off port for tank 13 is provided by the pipe and flange assembly 23 which communicates with the interior space 24 of tank 13.

In falling strand devolatilizer 11, the vessel 14 is thus positioned spatially above tank 13 and is generally coaxial therewith. The top portion 15 of vessel 14 conveniently meets at its rim or edge portion 26 with the upper end of the side walls 21 of tank 13, the vessel 14 being secured to the tank 13 by any convenient means such as welding or the like so that the vessel 14 is sealingly engaged with the tank 13 at rim portion 26. A material input port for vessel 14 is provided by pipe and flange assembly 27 which communicates with the interior space 19 of vessel 14. The tapered lower region 22 of tank 13 ends in a discharge port (not shown) which interconnectswith pipe and flange assembly 28, the pipe and flange assembly 28 communicating with the interior space 24 of the vessel 13 (the tube input and output means for circulating heat exchange fluid in the respective double walls of vessel 14 and tank 13 are not shown, nor are the pipe means for inputting and outputting heat exchange fluid for the heat exchanger assembly l2).

The heat exchanger assembly 12 has its tube portions (not shown) interconnect by suitable pipe means 29 with the pipe and flange assembly 27 of vessel 14. As those skilled in the art will appreciate, the heat exchanger assembly 12 is itself composed of two main subassemblies, a shell portion and a tube core, all of conventional construction. Into header 31 of the shell and tube heat exchanger 12 is input a process fluid (not shown) for heating, while from header 32, the thus heated process fluid exits from the tubes within the heat exchanger assembly 12 and enters the pipe 29.

Process fluid leaving tank 13 through pipe and flange assembly 28 enters a pumping screw assembly 33 from which the process fluid is pumped as through a pipe 34 for subsequent processing.

The discharge port 17 of vessel 14 forms the input port for the vessel or tank 13. The tapered lower region 16 of vessel 14 interconnects with a pipe 36 at port 17. Pipe 36 serves as a valve plate means extending generally circumferentially about port 17.

A valve disc 37 ispositioned generally across port 17 but within the neck of pipe 36. Valve disc 37 is positioned and adapted for rotational movements about a generally horizontal axis. When the valve disc 37 is generally in a horizontal position across port 17, the valve disc 37 substantially shuts port 17. Particularly when the apparatus of this invention is to be used for the processing of viscous fluids using elevated temperatures, it is preferred that the circumferential edge region of the valve disc 37 not engage the pipe 36 but rather be in slighly spaced relationship thereto, the spacing between the edge of the valve disc 37 and the adjacent inner wall of pipe 36 being sufficiently small as to substantially prevent any passage therethrough of viscous fluid during operation of an assembly of the invention. i

A valve stem38 extends generally coaxially along the indicated horizontal turning axis of the valve disc 37. The valve stem 38 projects beyond opposed side edges of the valve disc 37 through appropriate port means defined in the pipe 36. The portions of valve stem 38 thus adjacent to pipe 36 are journaled in bearing means (not shown) positioned in supports 39 and 40 which depend from the wall of tapered lower region 16 of vessel 14. These bearings in respective supports 39 and 40 permit revolvable movement of the valve disc 37 on the valve stem 38. I

One end portion of the valve stem 38 projects laterally outwardly to a terminal location which is exteriorly located with respect to the vessel 14 and which is also exteriorly located with respect to the tank 13 in this embodiment 10. In the region where the valve stem 38 extends through the side wall 21 of tank 13, sealing means between such side wall 21 and valve stem 38 is provided (not shown); When embodiment 11 is in operation, the level of fluid material in the bottom of vessel 14 overvalve stem 38 is such as to act as a sealing means between valve stem 38 and pipe 36 thereby permitting maintenance of a differential pressure between the respective spaces 19 and 24.

An actuator means, herein designated in its entirety by the numeral 42, is provided to move the valve stem 38 rotationally in response to signal means applied to actuator means 42. Actuator means 42 is conveniently mounted by some sort of bracket means (not shown) in fixed relationship to tank 13. The rotatable shaft of actuator 42 conveniently interconnects with the valve stem 38 by means of a collar assembly or the like (not shown). Any convenient conventional type of actuator' can be employed. .The falling strand devolatilizer l1 illustrates a preferred embodiment of the present invention where the actuator means 42 is completely outside of the process zones (here, the interior space 19 of vessel l4 and the interior space 24 of tank 13.)

In order to measure process fluid level in the vessel 14, preferably in the lower most region thereof, a level sensor as shown in FIG. 2 is employed. Level sensed by the level sensing means is converted to a signal output as by a transmitter 43 or the like, the signal being generated by transmitter 43 being representative of the fluid level measured inside the vessel 14 in an operating devolatilizer 11. In the embodiment 10, the level sensing means preferably comprises a radiation source and a radiation detector, although any convenient level sensing means, as those skilled in the art will appreciate, may be used to measure level in the vessel 14 in the practice of the present invention.

A controller 44 functionally interconnects the level sensing means and the actuator means 42. Controller 44 is adapted, in response to signals from the level sensing means, to generate control signals for the actuator means 42. Thus, rotational movements of the valve stem 38 are controlled and the valve disc 37 engaged therewith is open or closed to a predetermined extent in relation to the pipe Thus, a predetermined variable aperture is maintainable between the pipe 36 and the valve disc 37.

In space 24 is shown positioned an optional perforated basin-like structure 46 which is here suspended in fixed relationship to port 17 by means of rods 47 and 48 which interconnect in turn with the supports 39 and 40, respectively. While this assembly does not constitute part of the present invention, when during apparatus operation process fluid leaving pipe 36 falls into basin-like, structure 46, the perforations in basin-like structure 46 permit a viscous process fluid to be stranded into a plurality of high surface area components in the interior space 24 of vessel 13. When such a structure 46 is employed, independent suspending means therefor are preferably used.

The interrelationship between the level sensing I means, the actuator means 42, and the controller 44, is depicted in block diagrammatic form in FIG. 2. Thus,

gamma radiation emitted from radiation source is detected by a radiation detector. A signal from the detector is fed to a transducer where a signal output is generated which is representative of variations in fluid level in the vessel 14, thereby to complete level measurement. The output from the transducer is fed to a level controller where'the level measurement signal is compared to a predetermined level set point and an output signal is generated which is fed to the actuator means associated with the valve stem 38, the whole being reference. in FIG. 3 as the level control valve.

I Turning to FIGS. 3 and 4, there is seen another embodiment of the present invention. This embodiment is generally designated in its entirety by the numeral 51 and it is incorporated into a falling strand devolatilizer assembly which is herein referred to in its entirety by the numeral 52. Devolatilizer 52 can be considered to comprise a shell and tube heat exchanger assembly 53, and vessel 54 or first flash tank (which comprises part of the embodiment 51, and a second flash tank 56). In the shell and tube heat exchanger assembly 53, process fluid (not shown) is input to the tubes 57 of exchanger assembly 53 through header 58 and exits from the heat exchanger assembly 53 through header 59. The shell portion 61 of heat exchanger assembly 53 is used for circulation of a heat exchange fluid (not shown) which enters the shell 61 as through a pipe 62, circulates within the shell 61 about tubes 57 and exits from .the shell 61 asv through a pipe 63. The heat exchange fluid is conveniently heated in a heater .assembly 64. The temperature of the heat exchange fluid entering the shell 61 through pipe 62 is conveniently regulated by means of a temperature control means 66 which is conventional in construction. From the header 59, the process fluid is conducted into the vessel 54 via pipe means 67 where vapor separated from the process fluid is removed by means of the pipe 68, the pipe 68 being interconnected with a vapor recovery means (not shown) and vacuum pump means (not shown). A valve assembly, herein designated in its entirety by the numeral 69 at the lower end of the tank or vessel 54 meters process fluid at a controlled rate from the vessel 54 into the vessel 56 through the longitudinally expandable pipe 71. In the tank 56, process fluid has further volatiles separated therefrom which are removed through a pipe 72, the pipe 72 being connected to a vapor recovery means (not shown) and vacuum pump means, (not shown).

Process fluid is removed from the second flash tank 56 bottom portion through a melt pump 73.

Constructional details of the valve assembly 69 are illustrated in FIG. 4. Across an upper section of pipe 71 extends a valve disc 74 being adapted for rotational movements about a generally horizontal axis, the valve disc 74 is adapted to substantially shut the pipe 71 when the valve disc 74 is in a relatively horizontal position thereto. Associated with valve disc 74 are valve stems 76 and 77 which protrude in diametrically opposed fashion from valve disc 74 on opposite side edges thereof. Stem 76 is journaled for rotational movements in the bearing assembly 78, while stem 77 is journaled for rotational movements in bearing assembly 79. Stems 76 and 77 adapt the valve disc 74 for rotational movements. Bearings 78 and 79 are mounted in a ring 81. Ring 81 is adapted to be mounted across pipe 71 between a pair of flanges 82 and 83 as by 'means of nut and bolt assemblies 84 or the like. The valve assembly 69 is an unlined, all metal assembly suitable for utilization in the characteristically strenuous operating environments associated with, for example, wiped film devolatilizers used for devolatilizing polystyrene. Stem 76 extends through the ring 81 radially to a terminal location exteriorly positioned with respect to the ring 81. A seal 86 between the stem 76 and the ring 81 is provided to prevent ingress of air to the evacuated process space. Gaskets (not shown) on opposed faces of ring 81 seal the ring 81 between the adjacent faces of flanges 82 and 83. in the valve assembly 69, the ring 81 may I be regarded as a valve plate.

The stem 76 is connected at its exterior end region with a variable actuator means 87, the actuator 87 being exteriorly located in a fixed position relative to the vessel 54 and functionally adapted to rotatably move the valve stem 76 in response to signal means applied thereto.

Level sensing means associated with the vessel 54 (preferably the lower regions thereof) is provided. Level sensing means 88 is adapted both to measure a fluid level in the vessel 54 and to generate a signal means in response thereto when the vessel 54 is functioning in the falling strand devolatilizer 52 with the valve assembly 69 at the downwardly extending end of the vessel 54. I

A level controller 89 functionally interconnects the level sensing means 88 and the actuator means 87. The level controller means 89 is adapted, in response to signals from the level sensing means 88, to generate control signals for the actuator means 87, thereby to control rotational movements of the valve stem 76 and 77 in a predetermined manner. Thus, a predetermined variable aperture is maintainable between the valve assembly ring 81 and the valve disc 74.

Other and further embodiments of the present invention will be apparent to those skilled in the art from a reading of the present specification and drawings.

What is claimed is:

1. A flash tank and integral valve assembly adapted for the devolatilization, storage and continuously controlled transfer therefrom of viscous fluid and comprising in combination:

A. a generally enclosed vessel having adjacent one end region thereof a generally funnel-shaped configuration terminating in a discharge port means at said one end region said vessel being adapted to be oriented spatially with said one end region downwardly when said vessel is operating, said vessel having defined therein at least two additional ports in an upper wall region thereof, one port being adapted as a fluid input port, a second port being adapted as a vapor removal port, said vapor re moval port interconnecting with a vacuum pump means adapted to remove volatiles from said viscous fluid and said vessel,

B. valve disc means positioned generally across said discharge port means and adapted for rotational movements therein about a generally horizontal axis when said vessel is operating, said valve disc means further being adapted to substantially shut said port means when in a generally horizontal position relative thereto when said vessel is operating,

C. valve stem means extending generally coaxially with said horizontal axis and projecting beyond opposed side edges of said valve disc means into generally mating openings defined in a valve plate means, said valve stem means including bearing means, one end portion of said valve stem means projecting laterally outwardly to a terminal location exteriorly located with respect to said vessel, said valve stem means being adapted to rotatably move said valve disc means about said horizontal axis between shut and open positions relative to said port means, said bearing means having a seal between said valve stem means and said valve plate means, thereby preventing the ingress of air to the evacuated process space, v v

D. variable actuator means exteriorly located with respect to said vessel and functionally engaged with said exposed terminal end portion of said valve stem means, said actuator means being adapted in response to signal means applied thereto to move revolvably said valve stem means,

E. level sensing means associated with said vessel and adapted both to measure a fluid level'in' said vessel and to generate a signal means in response thereto when said vessel is functioning with said one end region downwardly oriented spatially, wherein said level sensing means comprises a gamma ray emitter located adjacent said one end region of said vessel exteriorly thereof, a gamma ray sensor located adjacent said one end region of said vessel exteriorly thereof but on an opposite side of said vessel relative to said emitter, and correlator means adapted to make said sensor and said emitter adapted to detect variations in said fluid level and to generate a signal means responsive thereto, and

F. controller means functionally interconnecting said level sensing means and said actuator means and adapted in response to signals from said level sensing means to generate control signals for said actuator means to control rotational movements of said valve stem means and said valve disc means engaged therewith in a predetermined manner whereby a predetermined variable aperture is maintainable between said discharge port means and said valve disc means.

2. The assembly of claim 1 wherein said vessel interconnects with another vessel at said one end region below said discharge port means.

3. The assembly of claim 2 wherein said vessel interconnects with another vessel'at said one end region below said valve plate means and said other vessel has defined therein at least two additional ports, one of which is connected with a source of pressure lower than that maintained by said pump means in said vessel. 

1. A flash tank and integral valve assembly adapted for the devolatilization, storage and continuously controlled transfer therefrom of viscous fluid and comprising in combination: A. a generally enclosed vessel having adjacent one end region thereof a generally funnel-shaped configuration terminating in a discharge port means at said one end region said vessel being adapted to be oriented spatially with said one end region downwardly when said vessel is operating, said vessel having defined therein at least two additional ports in an upper wall region thereof, one port being adapted as a fluid input port, a second port being adapted as a vapor removal port, said vapor removal port interconnecting with a vacuum pump means adapted to remove volatiles from said viscous fluid and said vessel, B. valve disc means positioned generally across said discharge port means and adapted for rotational movements therein about a generally horizontal axis when said vessel is operating, said valve disc means further being adapted to substantially shut said port means when in a generally horizontal position relative thereto when said vessel is operating, C. valve stem means extending generally coaxially with said horizontal axis and projecting beyond opposed side edges of said valve disc means into generally mating openings defined in a valve plate means, said valve stem means including bearing means, one end portion of said valve stem means projecting laterally outwardly to a terminal location exteriorly located with respect to said vessel, said valve stem means being adapted to rotatably move said valve disc means about said horizontal axis between shut and open positions relative to said port means, said bearing means having a seal between said valve stem means and said valve plate means, thereby preventing the ingress of air to the evacuated process space, D. variable actuator means exteriorly located with respect to said vessel and functionally engaged with said exposed terminal end portion of said valve stem means, said actuator means being adapted in response to signal means applied thereto to move revolvably said valve stem means, E. level sensing means associated with said vessel and adapted both to measure a fluid level in said vessel and to generate a signal means in response thereto when said vessel is functioning with said one end region downwardly oriented spatially, wherein said level sensing means comprises a gamma ray emitter located adjacent said one end region of said vessel exteriorly thereof, a gamma ray sensor located adjacent said one end region of said vessel exteriorly thereof but on an opposite side of said vessel relative to said emitter, and correlator means adapted to make said sensor and said emitter adapted to detect variations in said fluid level and to generate a signal means responsive thereto, and F. controller means functionally interconnecting said level sensing means and said actuator means and adapted in response to signals from said level sensing means to generate control signals for said actuator means to control rotational movements of said valve stem means and said valve disc means engaged therewith in a predetermined manner whereby a predetermined variable aperture is maintainable between said discharge port means and said valve disc means.
 2. The assembly of claim 1 wherein said vessel interconnects with another vessel at said one end region below said discharge port means.
 3. The assembly of claim 2 wherein said vessel interconnects with another veSsel at said one end region below said valve plate means and said other vessel has defined therein at least two additional ports, one of which is connected with a source of pressure lower than that maintained by said pump means in said vessel. 